Known in the art is an internal combustion engine which is provided with a variable compression ratio mechanism which can make a cylinder block move along a cylinder axis with respect to a crankcase so as to change the mechanical compression ratio. In general, the lower the engine load, the lower the thermal efficiency, so in such an internal combustion engine which is provided with a variable compression ratio mechanism, the mechanical compression ratio is made higher the lower the engine load so as to raise the expansion ratio and raise the thermal efficiency.
In this way, in an internal combustion engine which is provided with a variable compression ratio mechanism, target mechanical compression ratios are respectively set for the current engine operating states and the variable compression ratio mechanism is controlled so that the current target mechanical compression ratios are realized. However, in actuality, sometimes the current target mechanical compression ratio is not realized. If the target mechanical compression ratio is not realized, the current desired expansion ratio is also not realized.
In this way, it is desirable to estimate the current actual mechanical compression ratio. For example, the cylinder pressure at the time of combustion is affected by the amount of fed fuel, so it has been proposed to use the cylinder pressure at top dead center during a fuel cut operation as the basis to estimate the current actual compression ratio (see PLT 1).